A pupillary distance (PD) between pupils of a user may be measured, e.g., in addition to the refractive prescription for eyeglasses, e.g., mono-focal or multi-focal eyeglasses.
Optical centers of eyeglasses may be configured to coincide with a line of sight of the user, for example, to provide clear and convenient vision.
Multi-Focal (MF) spectacles, which may have a narrow distant vision zone, may require higher accuracy in the PD measurement than mono-focal spectacles.
The PD may be stated as two unequal numbers of distances from a frame center, e.g., a center of a nose of the user, for example, if symmetry in the PD is not always a constitution, e.g., in strabismus cases.
Discrepancy in the pupillary distance may lead, for example, to double vision, headaches, and/or other unwanted effects.
A degree of a possible error in the pupillary distance may depend on the power of the lens, e.g., an Rx of spectacles. For example, for a low power of the lens, larger errors in the pupillary distance may not affect a vision of the user.
An error tolerance of the pupillary distance may not be symmetric. In one example, if a measured PD of a user is less than an actual PD of the user, e.g., a negative error, the user may be able to compensate for the negative error, for example, by a slight accommodation of the eyes, which may lead to eye convergence that may reduce the actual PD of the user. In another example, a measured PD of a user, which is larger than an actual PD of the user, e.g., a positive error, may result in some degree of double vision and/or other inconveniences.